Lithographic apparatus, device manufacturing method and device manufactured thereby

ABSTRACT

The X, Y and Rz positions of a mask stage are measured using two optical encoder-reading heads measuring displacements of respective grid gratings mounted on the mask stage. The grid gratings are preferably provided on cut-away portions of the mask table so as to be coplanar with the pattern on the mask itself. Measurements of the table position in the other degrees of freedom can be measured with capacitive or optical height sensors.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation application of U.S. patentapplication Ser. No. 11/293,508 filed on Dec. 5, 2005 (that issued asU.S. Pat. No. 7,561,270 on Jul. 14, 2009), which is acontinuation-in-part application of U.S. patent application Ser. No.10/825,215, filed on Apr. 16, 2004 (that issued as U.S. Pat. No.7,289,212 on Oct. 30, 2007), which is a continuation-in-part of U.S.patent application Ser. No. 09/928,462, filed on Aug. 22, 2001 (thatissued as U.S. Pat. No. 6,819,425 on Nov. 16, 2004), which claimspriority to EP 00307306.1 filed on Aug. 24, 2000, the contents of whichare incorporated herein by reference in their entirety. Also, relatedsubject matter is disclosed in U.S. patent application Ser. No.12/194,789 filed on Aug. 20, 2008 (that issued as U.S. Pat. No.7,633,619 on Dec. 15, 2009), and is a divisional application of U.S.patent application Ser. No. 11/293,508 filed on Dec. 5, 2005 (thatissued as U.S. Pat. No. 7,561,270 on Jul. 14, 2009), the contents ofwhich are incorporated herein by reference in their entirety.

BACKGROUND

1. Field of Invention

The present invention relates generally to lithographic projectionapparatus.

2. Related Art

The term “patterning device” as here employed should be broadlyinterpreted as referring to devices that can be used to endow anincoming radiation beam with a patterned cross-section, corresponding toa pattern that is to be created in a target portion of the substrate;the term “light valve” can also be used in this context. Generally, thepattern will correspond to a particular functional layer in a devicebeing created in the target portion, such as an integrated circuit orother device (see below). Examples of such patterning devices include:

-   -   A mask. The concept of a mask is well known in lithography, and        it includes mask types such as binary, alternating phase-shift,        and attenuated phase-shift, as well as various hybrid mask        types. Placement of such a mask in the radiation beam causes        selective transmission (in the case of a transmissive mask) or        reflection (in the case of a reflective mask) of the radiation        impinging on the mask, according to the pattern on the mask. In        the case of a mask, the support structure will generally be a        mask table, which ensures that the mask can be held at a desired        position in the incoming radiation beam, and that it can be        moved relative to the beam if so desired.    -   A programmable mirror array. An example of such a device is a        matrix-addressable surface having a viscoelastic control layer        and a reflective surface. The basic principle behind such an        apparatus is that (for example) addressed areas of the        reflective surface reflect incident light as diffracted light,        whereas unaddressed areas reflect incident light as undiffracted        light. Using an appropriate filter, the undiffracted light can        be filtered out of the reflected beam, leaving only the        diffracted light behind; in this manner, the beam becomes        patterned according to the addressing pattern of the        matrix-addressable surface. The required matrix addressing can        be performed using suitable electronic elements. More        information on such mirror arrays can be gleaned, for example,        from U.S. Pat. No. 5,296,891 and U.S. Pat. No. 5,523,193, which        are incorporated herein by reference. In the case of a        programmable mirror array, the support structure may be embodied        as a frame or table, for example, which may be fixed or movable        as required.    -   A programmable LCD array. An example of such a construction is        given in U.S. Pat. No. 5,229,872, which is incorporated herein        by reference. As above, the support structure in this case may        be embodied as a frame or table, for example, which may be fixed        or movable as required.

For purposes of simplicity, the rest of this text may, at certainlocations, specifically direct itself to examples involving a mask andmask table; however, the general principles discussed in such instancesshould be seen in the broader context of the patterning devices ashereabove set forth.

Lithographic projection apparatus can be used, for example, in themanufacture of integrated circuits (ICs). In such a case, the patterningdevice may generate a circuit pattern corresponding to an individuallayer of the IC, and this pattern can be imaged onto a target portion(e.g. comprising one or more dies) on a substrate (silicon wafer) thathas been coated with a layer of radiation-sensitive material (resist).In general, a single wafer will contain a whole network of adjacenttarget portions that are successively irradiated via the projectionsystem, one at a time. In current apparatus, employing patterning by amask on a mask table, a distinction can be made between two differenttypes of machine. In one type of lithographic projection apparatus, eachtarget portion is irradiated by exposing the entire mask pattern ontothe target portion at once; such an apparatus is commonly referred to asa wafer stepper. In an alternative apparatus—commonly referred to as astep-and-scan apparatus—each target portion is irradiated byprogressively scanning the mask pattern under the projection beam in agiven reference direction (the “scanning” direction) while synchronouslyscanning the substrate table parallel or anti-parallel to thisdirection; since, in general, the projection system will have amagnification factor M (generally <1), the speed V at which thesubstrate table is scanned will be a factor M times that at which themask table is scanned. More information with regard to lithographicdevices as here described can be gleaned, for example, from U.S. Pat.No. 6,046,792, incorporated herein by reference.

In a manufacturing process using a lithographic projection apparatus, apattern (e.g. in a mask) is imaged onto a substrate that is at leastpartially covered by a layer of radiation-sensitive material (resist).Prior to this imaging step, the substrate may undergo variousprocedures, such as priming, resist coating and a soft bake. Afterexposure, the substrate may be subjected to other procedures, such as apost-exposure bake (PEB), development, a hard bake andmeasurement/inspection of the imaged features. This array of proceduresis used as a basis to pattern an individual layer of a device, e.g. anIC. Such a patterned layer may then undergo various processes such asetching, ion-implantation (doping), metallization, oxidation,chemo-mechanical polishing, etc., all intended to finish off anindividual layer. If several layers are required, then the wholeprocedure, or a variant thereof, will have to be repeated for each newlayer. Eventually, an array of devices will be present on the substrate(wafer). These devices are then separated from one another by atechnique such as dicing or sawing, whence the individual devices can bemounted on a carrier, connected to pins, etc. Further informationregarding such processes can be obtained, for example, from the book“Microchip Fabrication: A Practical Guide to Semiconductor Processing”,Third Edition, by Peter van Zant, McGraw Hill Publishing Co., 1997, ISBN0-07-067250-4, incorporated herein by reference.

For the sake of simplicity, the projection system may hereinafter bereferred to as the “lens”; however, this term should be broadlyinterpreted as encompassing various types of projection system,including refractive optics, reflective optics, and catadioptricsystems, for example. The radiation system may also include componentsoperating according to any of these design types for directing, shapingor controlling the projection beam of radiation, and such components mayalso be referred to below, collectively or singularly, as a “lens”.Further, the lithographic apparatus may be of a type having two or moresubstrate tables (and/or two or more mask tables). In such “multiplestage” devices the additional tables may be used in parallel, orpreparatory steps may be carried out on one or more tables while one ormore other tables are being used for exposures. Twin stage lithographicapparatus are described, for example, in U.S. Pat. No. 5,969,441 and WO98/40791, incorporated herein by reference.

One of the most challenging requirements for micro-lithography for theproduction of integrated circuits as well as liquid crystal displaypanels is the positioning of tables. For example, sub-100 nm lithographydemands substrate- and mask-positioning stages with dynamic accuracy andmatching between machines to the order of 1 nm in all 6 degrees offreedom (DOF), at velocities of up to 2 ms⁻¹.

A popular approach to such demanding positioning requirements is tosub-divide the stage positioning architecture into a coarse positioningmodule (e.g. an X-Y table or a gantry table) with micrometer accuraciesbut traveling over the entire working range, onto which is cascaded afine positioning module. The latter is responsible for correcting forthe residual error of the coarse positioning module to the last fewnanometers, but only needs to accommodate a very limited range oftravel. Commonly used actuators for such nano-positioning includepiezoelectric actuators or voice-coil type electromagnetic actuators.While positioning in the fine module is usually effected in all 6 DOF,large-range motions are rarely required for more than 2 DOF, thus easingthe design of the coarse module considerably.

The micrometer accuracy required for the coarse positioning can bereadily achieved using relatively simple position sensors, such asoptical or magnetic incremental encoders. These can be single-axisdevices with measurement in one DOF, or more recently multiple (up to 3)DOF devices such as those described by Schaffel et al. “Integratedelectro-dynamic multicoordinate drives”, Proc. ASPE Annual Meeting,California, USA, 1996, p. 456-461. Similar encoders are also availablecommercially, e.g., position measurement system Type PP281R manufacturedby Dr. J. Heidenhain GmbH. Although such sensors can providesub-micrometer level resolution without difficulty, absolute accuracyand in particular thermal stability over long travel ranges are notreadily achievable.

Position measurement for the mask and substrate tables at the end of thefine positioning module, on the other hand, has to be performed in all 6DOF to sub-nanometer resolution, with nanometer accuracy and stabilityover the entire working range. This is commonly achieved usingmulti-axis interferometers to measure displacements in all 6 DOF, withredundant axes for additional calibration functions (e.g. calibrationsof interferometer mirror flatness on the substrate table).

Although the technology behind such interferometer systems is verymature, their application is not without problems. One of the mostsignificant drawbacks of the interferometer is the dependence ofwavelength on environmental pressure and temperature, as described bySchellekens P. H. J. “Absolute measurement accuracy of technical laserinterferometers” Ph.D. Thesis, T U Eindhoven, 1986, which is given by:

$\begin{matrix}{{\lambda_{a} = \frac{\lambda_{v}}{\eta}}{{where}\text{:}}} & (1) \\{{( {\eta - 1} )_{P,T,H,C} = {\frac{D \times 0.104126 \times {10^{- 4} \cdot P}}{1 + {0.3671 \times {10^{- 2} \cdot T}}} - {0.42066 \times {10^{- 9} \cdot H}}}}{D = {0.27651754 \times 10^{- 3} \times \lbrack {1 + {53.5 \times 10^{- 8}( {C - 300} )}} \rbrack}}} & (2)\end{matrix}$P: atmospheric pressure [Pa]T: atmospheric temperature [° C.]H: water vapor pressure [Pa]C: CO₂ content [ppm]

This remains one of the major problems in the thermal design of anoptical lithography system. Typically, both temperature and pressurealong the optical path of the interferometer has to be activelycontrolled to mK and mbar levels by the use of dry, clean (to betterthan Class 1) air, e.g. supplied by air showers.

In addition, the mounting adjustment of multi-axis interferometers fororthogonality and coplanarity, as well as the subsequent calibrationprocedure to remove any residual errors, are both extremely complex andtime consuming. Even after such adjustments and calibration procedures,the measurement is only accurate if the relative positions of theinterferometer blocks remain stable. The nanometer dimensional stabilityrequirements of the metrology frame, on which the interferometer blocksare mounted, imply that the metrology frame has either to be made out ofa material with low or zero coefficient of thermal expansion (CTE), suchas Invar or Zerodur, or active thermal stabilization to mK levels, orboth. Furthermore, the pointing stability of the laser beam duringoperation may introduce additional cosine or Abbe errors which need tobe calibrated out on a regular basis by some form of automated routine.

An interferometer system is of course only a relative measuring system,capable of measuring changes in length (of optical path, to be precise).A zero reference in each degree of freedom can only be generated withadditional equipment, such as so-called alignment sensors as describedin WO 98/39689.

Although metrology frames in state-of-the-art lithography systems arehighly isolated from ambient vibration, thermal deformation of the orderof 0.5.times.10.sup.−9 m is not totally avoidable. It is, therefore,desirable that the position of the substrate or mask tables be measureddirectly relative to the optical imaging system. Mounting ofinterferometers directly on the lens, for example, is both difficult andundesirable. Relative length measurement to the lens can, however, stillbe realized by differential interferometry, at the expense of the addedcomplication and cost.

The multiple beams required for such 6 DOF interferometric measurementcannot be adequately supplied with sufficient optical power by one lasersource, thus requiring multiple sources with additional wavelengthmatching demands. The total thermal dissipation of the lasers anddetectors combined exceeds 50 W, which is well above the level allowablefor the dimensional stability of the metrology frame. Both the lasersand the detectors have thus to be mounted remotely via optical links.

As can be seen, whilst the resulting interferometry based system istechnically viable and has been implemented in practice, it is by nomeans simple, robust and economical.

The most obvious alternative to interferometers for long-rangedisplacement measurements with micrometer or nanometer resolutions isthe optical incremental encoder. Optical encoders with sub-nanometerresolutions have become available in recent years and have been promotedas viable alternatives to single-axis interferometry. The sub-nanometerresolution is achieved by using fine-pitched gratings (down to 512 nn)in combination with interpolation techniques (up to 4096×). Most of suchencoders, however, provide length measurement in 1 DOF only. As such,they do not lend themselves readily to nano-metrology in all 6 DOFsimultaneously. Amongst the difficulties is the high level of crosstalkof the displacement signal to parasitic movements in the other 5 DOF.

SUMMARY

It is an object of the invention to provide an improved displacementmeasuring system for use in a lithographic projection apparatus, andespecially a system in which problems suffered by existing systems aresolved or ameliorated.

According to the invention there is provided a lithographic projectionapparatus comprising:

a radiation system for providing a projection beam of radiation;

a support structure for supporting a patterning device, the patterningdevice serving to pattern the projection beam according to a desiredpattern;

a substrate table for holding a substrate;

a projection system for projecting the patterned beam onto a targetportion of the substrate; characterized by:

a displacement measuring system for measuring the position of a moveableobject comprising one of the support structure and the substrate tablein at least two degrees of freedom, the displacement measuring systemcomprising at least one grid grating mounted on the moveable object andat least one sensor head for measuring displacements of the grid gratingin two degrees of freedom.

The invention also provides a lithographic projection apparatuscomprising:

a radiation system for providing a projection beam of radiation;

a support structure for supporting a patterning device, the patterningdevice serving to pattern the projection beam according to a desiredpattern;

a substrate table for holding a substrate;

a projection system for projecting the patterned beam onto a targetportion of the substrate; and:

a displacement measuring system for measuring the position of a moveableobject comprising one of the support structure and the substrate tablein at least two degrees of freedom, the displacement measuring systemcomprising at least one grid grating mounted on a reference frame and atleast one sensor head mounted on the moveable object for measuringdisplacement of the moveable object relative to the grid grating in twodegrees of freedom.

A major advantage of the 2D grid encoder is that the measurement gridcan be permanently fixed on a grating plate. Even if the grating is notperfectly orthogonal, straight or linear, this remains unchanged as longas the grating plate is free from distortions (either thermal orelastic). Such linearity or orthogonality errors can be calibrated outwithout too much difficulty by, for example, vacuum interferometry. Thecalibration only needs to be performed once for each grating, or not atall if one is only interested in positional repeatability. The use of agrid encoder essentially removes the guideway straightness andorthogonality from the error budget, when compared with single-axisencoder-based solutions.

The present invention can therefore provide an alternative solution tointerferometry, at least in 3 coplanar degrees of freedom (X, Y, Rz), bycombining the principles of grid gratings and sub-nanometer encoding.The invention can both be practiced for positional measurement of a maskstage or a substrate stage.

To address the issue of output sensitivity to parasitic movements in theremaining degrees of freedom of encoders with nanometer resolutions,systems used in the present invention make use of the interferencepattern of the first order diffraction of the collimated incidence lightfrom a monochromatic source off the grating. This method ensures thatthe signals at the detector are free from high-order harmonics, makingit possible to perform very high interpolation without incurringexcessive errors. In addition, it allows a much larger position latitudeof the reading head relative to the grating in the non-measurementdirections. For more information on such a detector see U.S. Pat. No.5,643,730, which document is hereby incorporated herein by reference.

A typical system used in the present invention comprises a grid gratingwith a period of 10 μm or less, with an interferential reading (encoder)head in 2 DOF and an interpolator of up to a factor of 20,000 for eachaxis.

For the measurement of the remaining 3 DOF, namely Z, Rx and Ry, variousshort range displacement sensing technologies can be employed, includingoptical triangulation, fiber optic back-scatter, interferometric sensors(which can have a very short optical path in air and therefore be muchless sensitive to environmental fluctuations), capacitive or inductivesensors.

Currently, capacitive and optical sensors are preferred to the othermeasuring principles, though the others may be appropriate in someapplications of the invention. The use of inductive sensors against aZerodur chuck is problematic, as conductive targets are required for thesensors. Pneumatic proximity sensors (air micrometer), on the otherhand, suffer from limited resolution and working distance, as well asexerting a finite force on the target.

Optical sensors, whether interferometric or triangulated, can bedesigned with a relatively large (a few millimeters) working distance,which helps to ease assembly tolerances. Compared to capacitive sensors,they usually have higher bandwidths, and can be configured as anabsolute distance sensor. As an absolute sensor, however, they do sufferfrom long-term stability problems due to mechanical drifts (thermal orotherwise) requiring periodic calibration.

Capacitive sensors, on the other hand, can be designed as an absolutesensor with very high stability. Furthermore, the distance measurementis performed over a relatively large target surface, which helps toreduce any effects of localized unevenness of the target surface.Despite their limited measurement range and stand-off clearance, theyare currently the preferred choice in lithographic applications.

An encoder based nano-positioning system offers an advantageousalternative to interferometry and is much simpler to implement. Bettermeasurement stability can be achieved by the fact that the measurementgrid in the X-Y plane is permanently fixed onto the mask table, whichwhen implemented in a zero-CTE material, such as Zerodur, is bothlong-term dimensionally stable and thermally insensitive. This easesconsiderably the stringent demand on environmental control of the areaimmediately around the optical path of the interferometer beams,particularly in the case of a lithographic projection apparatusemploying wavelengths of 157 nm or below. Such devices require to bepurged with gas, that does not absorb the beam (which is stronglyabsorbed in air), and by avoiding the need for air showers over thelength of the interferometer beams, the present invention cansubstantially reduce consumption of purge gas.

The mask position relative to the projection optics can also be measuredin the encoder solution without resorting to a differentialconfiguration. Although placing the reading head directly on the top ofthe projection optics does put more demands on the thermal dissipationof the former, techniques to minimize this such as active cooling orremote light source and detectors linked by optical fibers are alreadyavailable and already deployed in state-of-the-art interferometersystems.

The encoder based nano-positioning system may also be arranged asfollows: A first sensor head is arranged on the substrate table, and afirst grating is mounted on the projection system or on a frame. In oneembodiment, such a frame is at least substantially stationary relativeto the projection system, which means that the relative displacement ofthe projection system and the part of the frame the first grating isattached to is negligible. If this cannot be achieved, it is envisagedthat displacement of the frame relative to the projection system ismeasured as well. It is also envisaged that in case the frame is deemedto be substantially stationary, displacement of the frame relative tothe projection system is measured as well, in order to make sure thatthe relative displacement actually is negligible.

The first sensor head can be mounted on top of the substrate table,facing upwards to the first grating, which is mounted on the projectionsystem or on the frame. The sensor head and the first grating arearranged in such a way that during all movements of the substrate table,the first sensor head and the first grating are able to cooperate inmeasuring displacement of the substrate table relative to the projectionsystem.

In such an arrangement, the measurement system may also comprise a firstz-sensor. The z-sensor and the first grating cooperate in determiningthe displacement of the substrate table relative to the projectionsystem in a third direction of measurement which is perpendicular to aplane defined by the first direction of measurement and the direction ofthe lines of the first grating. For example, when the first direction ofmeasurement is the x-direction and the sensor head sends the polarizedbeam of radiation in z-direction to the first grating, the lines of thefirst grating extend in y-direction. In that case, the plane defined bythe first direction of measurement and the direction of the lines of thefirst grating is the x-y-plane. In that case, the third direction ofmeasurement is the direction perpendicular to the x-y-plane, thus thez-direction. So, in this situation the first grating is not only usedfor measuring displacement of the substrate table relative to theprojection system in x-direction, but also for measuring displacement ofthe substrate table relative to the projection system in z-direction.

It is envisaged that the z-sensor can be an interferometer, and that thefirst grating has a reflective face cooperating with the interferometer.It is possible that the grating comprises two, mutually parallelreflective faces, for example due to the nature of the grating. This canbe the case if the grating comprises two groups of parallel lines. Insuch a case, the distance between the reflective faces is preferably ntimes the wavelength of the beam of the interferometer, n being apositive integer (1, 2, 3, 4, . . . ).

Alternatively, the z-sensor can be a capacitive sensor. For functioning,a capacitive sensor needs an electrically conductive counter plate, thatis arranged at a small distance, such as a few millimeters, from thecapacitive sensor in the direction of measurement. By making at least apart of the first grating electrically conductive, the first gratingserves as a counter plate for the capacitive sensor. It is known to makegratings out of glass, with lines of chromium. By interconnecting thelines and connecting them to ground, such a grating can be used with acapacitive sensor.

In the lithographic projection system according to the invention, thenumber of interferometers used for measuring the displacements of thesubstrate table relative to the projection system is reduced. This way,the costs of both the measurement system and the substrate table arereduced.

It is envisaged that the displacements of the substrate table relativeto the projection system in both the first direction and the seconddirection are measured using sensor heads or reading heads. In thisembodiment, the lithographic apparatus is equipped with a measurementsystem comprising a first sensor head and a second sensor head. To thisend, the first grating may include not only a first group of mutuallyparallel lines, extending perpendicular to the first direction ofmeasurement and but also a second group of mutually parallel lines,extending perpendicular to the second direction of measurement. Thefirst grating may be provided with a checkerboard pattern, for use inassociation with both the first and the second sensor head. In thatcase, the transitions between the light and the dark areas of thecheckerboard pattern take over the role of the parallel lines of thegrating. Alternatively, a separate second grating may be present for useby the second sensor head. The first and the second sensor head arepreferably integrated in a single sensor unit.

In another embodiment, the measurement system is adapted to measuredisplacements of the substrate table relative to the projection systemin all three degrees of freedom in the plane defined by the first andthe second direction of measurement by using sensor heads. If this planeis the x-y-plane, this means that the translational displacements of thesubstrate table in the x-direction and in the y-direction and therotational displacement about an axis in z-direction are measured usingsensor heads.

For this purpose, the measurement system comprises in this embodiment afirst, a second and a third sensor head. Two of these sensor heads areused for measuring the displacements of the substrate table in the firstdirection of measurement and the other encoder is used for measuring thedisplacements of the substrate table in the second direction ofmeasurement. Of course, this can also be the other way around. Thesensor heads are arranged at known relative distances, so that anyrotation of the substrate table around an axis extending in the thirddirection of measurement can be determined based on the sensor headmeasurements.

It is possible that each sensor head is associated with its own grating,which means that in that case the measurement system comprises a first,a second and a third grating. However, in certain embodiments, at leasttwo sensor heads share a grating, which is suitable for measurements intwo directions, for example due to the presence of a checkerboardpattern.

It is envisaged that the measurement system for measuring displacementsof the substrate table relative to the projection system in all threedegrees of freedom in the plane defined by the first and the seconddirection of measurement using sensor heads as described above is usedwithout a z-sensor. In that case, the sensor heads are mounted onto thesubstrate table.

In a further alternative embodiment, the measurement system is providedwith a first, a second and a third z-sensor. These z-sensors arearranged at known relative positions, so that any rotation of thesubstrate table around the first direction of measurement and around thesecond direction of measurement can be determined by their measurements.Preferably all z-sensors cooperate with a grating in measuring thedisplacements of the substrate table relative to the projection system.This results in a measurement system capable of positional measurementin 6 degrees of freedom (6 DOF).

The invention also provides a device manufacturing method includingproviding a substrate that is at least partially covered by a layer ofradiation-sensitive material, providing a projection beam of radiationusing a radiation system; using a patterning device to endow theprojection beam with a pattern in its cross-section; projecting thepatterned beam of radiation onto a target portion of the layer ofradiation-sensitive material, and measuring displacements of one of thesupport structure and the substrate table in at least two degrees offreedom using at least one grid grating mounted thereon and at least onesensor head.

The invention further provides a method of calibrating a lithographicprojection apparatus including providing a reference pattern to apatterning device held in a moveable support structure, the referencepattern having a plurality of reference marks at pre-calibratedpositions in at least a scanning direction of the lithographicprojection apparatus, holding an image sensor on a substrate table at aconstant position relative to the projection lens, positioning thesupport structure so as to project an image of each of the referencemarks in turn onto the transmission image sensor, and measuring theposition of the support structure in at least a first degree of freedomwhen each of the reference marks is projected onto the image sensor.

In an embodiment, there is provided a lithographic projection apparatusincluding a support structure for supporting a patterning device, thepatterning device serving to pattern a beam of radiation according to adesired pattern; a substrate table for holding a substrate; a projectionsystem for projecting the patterned beam onto a target portion of thesubstrate; and a displacement measuring system for measuring a positionof a moveable object comprising one of the support structure and thesubstrate table, wherein the displacement measuring system includes anencoder system for measuring two degrees of freedom of the moveableobject relative to a reference frame, and wherein the displacementmeasurement system also includes a Z-sensor for measuring at leastanother degree of freedom of the moveable object relative to thereference frame.

In another embodiment of the invention, there is provided a lithographicprojection apparatus including a support structure for supporting apatterning device, the patterning device serving to pattern a beam ofradiation according to a desired pattern; a substrate table for holdinga substrate; a projection system for projecting the patterned beam ontoa target portion of the substrate; a reference frame comprising a lensframe including a lens of the projection system and a measurement frame,an encoder system for measuring displacements between the measurementframe and the lens frame; and a displacement measuring system formeasuring a position of a moveable object comprising one of the supportstructure and the substrate table with respect to the measurement frame,such that the position of the moveable object relative to the lens framecan be determined.

Although specific reference may be made in this text to the use of theapparatus according to the invention in the manufacture of ICs, itshould be explicitly understood that such an apparatus has many otherpossible applications. For example, it may be employed in themanufacture of integrated optical systems, guidance and detectionpatterns for magnetic domain memories, liquid-crystal display panels,thin-film magnetic heads, etc. The skilled artisan will appreciate that,in the context of such alternative applications, any use of the terms“reticle”, “wafer” or “die” in this text should be considered as beingreplaced by the more general terms “mask”, “substrate” and “exposurearea” or “target area”, respectively.

In the present document, the terms “radiation” and “beam” are used toencompass all types of electromagnetic radiation, including ultravioletradiation (e.g. with a wavelength of 365, 248, 193, 157 or 126 nm) andEUV (extreme ultra-violet radiation, e.g. having a wavelength in therange 5-20 nm), as well as particle beams, such as ion beams or electronbeams.

The invention is described below with reference to a coordinate systembased on orthogonal X, Y and Z directions with rotation about an axisparallel to the I direction denoted Ri. The Z direction may be referredto as “vertical” and the X and Y directions as “horizontal”. However,unless the context otherwise demands, this should not be taken asrequiring a specific orientation of the apparatus.

BRIEF DESCRIPTION OF THE DRAWINGS/FIGURES

The invention and its attendant advantages will be further describedbelow with reference to exemplary embodiments and the accompanyingschematic drawings, in which:

FIG. 1 depicts a lithographic projection apparatus according to a firstembodiment of the invention;

FIG. 2 is a perspective view of the mask stage of a known lithographicapparatus, showing the position measuring system;

FIG. 3 is a perspective view of the mask stage of a lithographicapparatus, showing the position measuring system according to a firstembodiment of the invention.

FIG. 4 shows a second embodiment of a lithographic apparatus accordingto the invention;

FIG. 5 shows a top view of the embodiment of FIG. 4;

FIG. 6 shows a third embodiment of a lithographic apparatus according tothe invention;

FIG. 7 shows a fourth embodiment of a lithographic apparatus accordingto the invention;

FIG. 8 a-b show possible arrangements of multiple sensor heads in asensor unit;

FIG. 9 shows a fifth embodiment of a lithographic apparatus according tothe invention;

FIG. 10 shows a sixth embodiment of a lithographic apparatus accordingto the invention;

FIG. 11 shows a seventh embodiment of a lithographic apparatus accordingto the invention;

FIG. 12 shows an eight embodiment of a lithographic apparatus accordingto the invention.

DETAILED DESCRIPTION

FIG. 1 schematically depicts a lithographic projection apparatusaccording to a particular embodiment of the invention. The apparatuscomprises:

a radiation system including a beam expander Ex, and an illuminationsystem IL, for supplying a projection beam PB of radiation (e.g. UVradiation), which in this particular case also comprises a radiationsource LA;

a first object table (mask table) MT provided with a mask holder forholding a mask MA (e.g. a reticle), and connected to first positioningmodule for accurately positioning the mask with respect to a projectionsystem PL;

a second object table (substrate table) WT provided with a substrateholder for holding a substrate W (e.g. a resist-coated silicon wafer),and connected to second positioning module for accurately positioningthe substrate with respect to item PL;

a projection system (“lens”) PL (e.g. lens group) for imaging anirradiated portion of the mask MA onto a target portion C (e.g.comprising one or more dies) of the substrate W. As here depicted, theapparatus is of a transmissive type (i.e. has a transmissive mask).However, in general, it may also be of a reflective type, for example(with a reflective mask). Alternatively, the apparatus may employanother kind of patterning device, such as a programmable mirror arrayof a type as referred to above.

The radiation source LA (e.g. an excimer laser) produces a beam ofradiation. This beam is fed into an illumination system (illuminator)IL, either directly or after having traversed conditioning opticalelements, such as a beam expander Ex, for example. The illuminationsystem (illuminator) IL may comprise adjustable optical elements AM forsetting the outer and/or inner radial extent (commonly referred to as.sigma.-outer and .sigma.-inner, respectively) of the intensitydistribution in the beam. In addition, it will generally comprisevarious other components, such as an integrator IN and a condenser CO.In this way, the projection beam PB impinging on the mask MA has adesired uniformity and intensity distribution in its cross-section.

It should be noted with regard to FIG. 1 that the radiation source LAmay be within the housing of the lithographic projection apparatus (asis often the case when the source LA is a mercury lamp, for example),but that it may also be remote from the lithographic projectionapparatus, the radiation beam which it produces being led into theapparatus (e.g. with the aid of suitable directing mirrors); this latterscenario is often the case when the source LA is an excimer laser. Thecurrent invention and Claims encompass both of these scenarios.

The projection beam PB subsequently intercepts the mask MA, which isheld on a mask table MT. Having traversed the mask MA, the projectionbeam PB passes through the lens PL, which focuses the projection beam PBonto a target portion C of the substrate W. With the aid of the secondpositioning module (and interferometer IF), the substrate table WT canbe moved accurately, e.g. so as to position different target portions Cin the path of the projection beam PB. Similarly, the first positioningmodule can be used to accurately position the mask MA with respect tothe path of the projection beam PB, e.g. after mechanical retrieval ofthe mask MA from a mask library, or during a scan. In general, movementof the object tables MT, WT will be realized with the aid of along-stroke module (course positioning) and a short-stroke module (finepositioning), which are not explicitly depicted in FIG. 1. However, inthe case of a wafer stepper (as opposed to a step-and-scan apparatus)the mask table MT may just be connected to a short stroke actuator, ormay be fixed.

The depicted apparatus can be used in two different modes:

1. In step mode, the mask table MT is kept essentially stationary, andan entire mask image is projected in one go (i.e. a single “flash”) ontoa target portion C. The substrate table WT is then shifted in the xand/or y directions so that a different target portion C can beirradiated by the projection beam PB;

2. In scan mode, essentially the same scenario applies, except that agiven target portion C is not exposed in a single “flash”. Instead, themask table MT is movable in a given direction (the so-called “scandirection”, e.g. the y direction) with a speed v, so that the projectionbeam PB is caused to scan over a mask image; concurrently, the substratetable WT is simultaneously moved in the same or opposite direction at aspeed V=Mv, in which M is the magnification of the lens PL (typically,M=¼ or ⅕). In this manner, a relatively large target portion C can beexposed, without having to compromise on resolution.

According to the first embodiment of the invention, the displacementmeasuring system for the mask table comprises a 6 DOF,non-interferometric, nano-metrology system which comprises a table madeof Zerodur, on the underside of which are exposed two grid gratings.Each of these gratings, one on either side of the table along theY-direction, has a measurement range of, for example, 10 mm.times.500mm. Two two-coordinate reading heads, mounted on either side of the lenstop, measure displacements of the mask table in X, Y, Rz with respect tothe lens, with redundant information for X. The two-dimensional gratingsare dimensionally stable to nanometer levels over a reasonabletemperature range due to the near-zero coefficient of thermal expansion(CCTE) of Zerodur, thus offering a ‘permanent’ frame of dimensionalreference. To minimize any Abbe error due to pitch and roll, the gratingshould preferably be coplanar to the patterned surface of the mask.Additional indexing channels in X, Y.sub.1 and Y.sub.2 can also beimplemented to provide zero references relative to the lens.

Displacements in the other 3 degrees of freedom (Z, Rx, Ry) can bemeasured by means of a minimum of 3 nano-height gauges. For theparticular case of a transmissive mask, the center portions of the lensand the mask table have to be kept clear. As such, a four-sensor layoutis more convenient for implementation.

Similar to displacement in the X-Y plane, it is more convenient to havethe sensing target (reflective surface for an optical sensor, orelectrode for a capacitive sensor) on the Zerodur table, and the activepart of the sensor on the lens top. This avoids, amongst other things,having to route sensor cables to the moving, nano-positioned mask table.The height gauges can use the 2D-grating as their targets, or separatetargets can be provided on the Zerodur table.

In contrast with the case of interferometers, the actual mountingpositions and orthogonalities of the sensor heads as well as the targetsurfaces are less critical as long as they remain stable, as they can bedetermined by a calibration procedure on the lithographic machine. Asalready mentioned, while laser interferometer is unrivalled in terms ofaccuracy, the incremental encoder is superior in terms of repeatabilitydue to the sensitivity of the former to environmental conditions.

Gratings for use in the present invention are preferably manufacturedusing a laser interferometer in a highly controlled environment (e.g.vacuum) to make a master encoder grating with the highest possibleaccuracy. Then production gratings are replicated from the master-takingadvantage of the encoder's inherently high repeatability. The replicascan further be calibrated, either against the master grating or againsta vacuum interferometer.

A crucial factor in the practicability of calibration is the spatialfrequency content of the errors. An encoder with high spatial-frequencyerrors will require a high density of calibration data, as well as ahigh-accuracy reference mark to initialize the application ofcorrections to measured position data.

Before describing a displacement measurement system according to thepresent invention, a conventional system will be outlined with referenceto FIG. 2 to emphasize the advantages of the present invention.

In the conventional system, the mask table MT has a relatively longrange of movement in the Y-direction to accommodate the scan of the maskduring the imaging process. Throughout this large-range motion, the Yposition of the mask table MT is measured using two Y1-IF, Y2-IF whichdirect measurement beams against one or more mirrors or reflectorsmounted on the mask table MT. The measurement beams are incident on themask table as two spaced-apart points so that the difference between thetwo resulting readings can be used to determine the Rz position of themask table. At least one extreme of the range of motion of the masktable, the measurement beams will extend over a considerable distanceand any variation in the refractive index of the atmosphere throughwhich they pass can therefore introduce a significant error into theposition measurements. The X position of the mask table is measured byX-interferometer X-IF. Although the range of motion of the mask table inthe X-direction is considerably smaller than that in the Y-direction, sothe optical path length of the X-interferometer does not need to be solong, the X-interferometer must provide a measurement of X positionthroughout the range of motion of the mask table in the Y-direction.This requires that the measurement beam of the X-interferometer X-IFmust be directed onto a mirror mounted on the side of the mask table MTand having a length greater than the scanning range of the mask tableMT.

In the conventional system, the three interferometers providemeasurements of displacements of the mask table in three degrees offreedom, namely X, Y and Rz (yaw). The position in the other threedegrees of freedom, i.e. Z, Rx (pitch) and Ry (roll), is provided byappropriate processing of the outputs from three height sensors HS whichmeasure the vertical position of three points spaced apart on the bottomof the mask table MT.

By way of comparison, the arrangement according to the first embodimentof the present invention is shown in FIG. 3. In place of theinterferometers Y1-IF, Y2-IF and X-IF, the present invention employs twooptical reading heads 10, 11 which measure displacements of respectivegrid gratings 12, 13. The grid gratings 12, 13 are provided one oneither side of the mask MA and have a length in the Y-directionsufficient to accommodate the entire scanning range of motion, indicatedby the double-headed arrow, of the mask table MT. The grid gratings 12,13 are positioned on cut-away portions so that they are substantiallyco-planar with the pattern on the mask MA. The encoder reading heads 10,11, as well as three height sensors HS, are mounted on, or fixedrelative to, the upper element of the projection system, represented bythe dashed oval in FIG. 3.

The encoder reading heads 10, 11 can be actively temperature-controlled,e.g. by incorporating a water-cooling jacket, to remove any heatdissipated by them and maintain thermal stability of the reading headitself and the projection optics to which they are mounted. Also, thelight source and the detectors of the reading head can be locatedremotely and coupled to the reading head via optical fibers, so as tominimize any local heat generation and maintain the highest possiblepointing stability in the reading head optics.

As can be seen from FIGS. 2 and 3, the encoder measurement system ismuch more compact, and removes the need for extending the metrologyreference frame from the wafer level to the reticle level, the two beingabout 1 meter apart in the vertical direction. The resultant design ofthe metrology frame is much simpler and more compact, with substantialimprovements in its dynamic characteristics. The concept can be takenfurther to measure the X-Y position of the mask itself relative to theprojection optics. This can be done by putting a reflective gratingdirectly on the chrome border around the pattern area of the mask. Whilethis increases the costs of the mask, any distortions in the plane ofthe mask due to e.g. dimensional changes during processing can beautomatically accounted for. The availability of a reference indexposition in X, Y and Rz is yet an additional bonus.

It is important that the encoder system of the present invention, whichis highly repeatable but not absolutely accurate, is calibrated to anabsolute length reference such as a vacuum interferometer. Thecalibration should ideally be carried in an offline calibration platformbut also in-situ in the machine for monitoring long-term drifts.

For offline calibration, the encoder grating, together with the readinghead, can be calibrated directly in the scan (Y) direction against alength reference system such as a vacuum interferometer. This is aone-time measurement and can be carried out outside the machine undercontrolled conditions. The error map so obtained can be stored in alook-up error table or as coefficients of polynomials in an errorcorrection routine implemented on the machine. The error map can becalibrated not only in the Y, but also in the X direction to account forany X-displacement dependence. Calibration in the transverse (X)direction is performed using a reference plane-mirror interferometersystem against a reference flat mirror, due to the long travel range inY.

To effect in-situ calibration in the apparatus it would be desirable toprovide a reference interferometer system but space requirementsgenerally prohibit this. Instead, a calibration scheme in which thecalibration in the scan direction is divided into three parts—the areaof the mask, over the area corresponding to the illumination field atboth ends of the mask and the rest of the range—is used.

In the central part of the movement range where the mask passes throughthe central line of the optical system, the encoder system can becalibrated using a reference mask (e.g. an ultra-flat mask made ofZerodur) on which a large number of reference marks are printed andexposed. The aerial images of the reference marks are detected by atransmission image sensor mounted on the substrate table WT which isheld relative to the reference frame RF, as illustrated in FIG. 1, in aconstant position using the wafer stage interferometer system IF.Meanwhile, the reference mask is moved in the mask stage to successivemarkers and the position of the encoder noted and compared to thepre-calibrated position of the marker on the reference mask. Theposition of the markers on the reference mask can be pre-calibrated toan absolute length standard, offline and on a regular basis.

The X-dependence of the encoder in the scan direction can also becalibrated by shifting the mask off-center, as well as the substratetable WT by an equivalent amount, and repeating the above procedure at aplurality of off-centered positions.

The range of movement immediately outside the mask area at both ends(equal to the size of the exposure slit at reticle level) is also ofcrucial importance to the overlay quality of the exposure. A highlyaccurate calibration in these areas can be obtained without includingthe error induced by lens distortion by shifting the substrate table WTsuch that the transmission image sensor catches at the outermostposition of the exposure field in the scan direction. The calibrationdescribed for the central area is then repeated over the entire lengthof the mask with transmission image sensor kept stationary relative tothe reference frame in the off-centered position. The error map producedby this procedure is combined to the original map at the center positionusing, for example, a least-square fitting routine.

Outside the above two ranges, where the mask table MT is in theacceleration or setting phase, the motion cycle has little impact on theactual overlay quality of the exposure and accuracy requirements aretherefore less stringent. High spatial frequency errors of the encodersystem in these regions can be calibrated by setting the mask table MTin motion at constant velocity and under low-bandwidth server controlsuch that the inertia of the mask table is used to filter out anyvelocity fluctuations. The velocity is then assumed constant and anyirregularity in the position data rate of the encoder system gives ameasure of any high frequency errors.

Calibration of the encoder system in the X direction can be carried outsimilarly using a number of markers on the reference mask along atransverse axis. For the calibration of the X dependence due to Ymovements, the mask table MT can be moved in the scan direction, usingthe now corrected Y axes to maintain constant yaw (Rz) and recording anycross talk of X position from Y using two X-measuring heads.

FIG. 4 and FIG. 5 show an embodiment of a lithographic apparatusaccording to the invention. Projection system 1 is adapted to project apatterned beam 4 of radiation onto a substrate 3. A substrate table 2 isprovided to carry the substrate 3. For monitoring displacement of thesubstrate table 2 relative to the projection system 1, a measurementsystem is provided.

In the embodiment of FIG. 4 and FIG. 5, the measurement system comprisesa sensor unit 9 and a first grating 20. The sensor unit 9 is mounted ontop of the substrate table 2. The sensor unit 9 cooperates with firstgrating 20, which is mounted onto the projection system 1 fordetermining displacement of the substrate table 2 in the x-y-planerelative to the projection system 1. The sensor unit 9 and the firstgrating 20 are arranged relative to each other in such a way that thesensor unit 9 and the first grating 20 are able to cooperate during allexpected movements of the substrate table 2 relative to the projectionsystem 1.

In the embodiment of FIG. 4 and FIG. 5, the measurement system uses anoptical encoder system for measuring displacement of the substrate table2 relative to the projection system 1. In general, optical encodersystems that are used for accurate displacement measurements comprise anoptical reading head that cooperates with a grating. The reading headsends a polarized first beam of radiation, such as a laser beam, to thegrating. The grating comprises a periodically alternating pattern, thepattern alternating in the direction of the measurement. As an example,the grating may comprise a group of mutually parallel lines, extendingsubstantially perpendicular to the direction of the beam. The gratingdivides the beam into a first order beam and a minus first order beam,and reflects those beams to a receiving unit in the sensor head. Whenthe first beam moves relative to the lines of the grating in a firstdirection of measurement, which is substantially perpendicular to boththe direction of the lines and the direction of the beam, a phase shiftoccurs in the reflected beams. Thus phase shift is used for determiningthe displacement of the first beam relative to the grating in the firstdirection of measurement.

In a sensor unit, a plurality of optical reading heads can be provided.Also, the first grating can be two-dimensional to make measurements inof displacement two or three degrees of freedom possible.

For measuring displacement of the substrate table 2 relative to theprojection system 1 in a first direction of measurement whichcorresponds to displacement in one translational degree of freedom, thesensor unit 9 comprises a first reading head 8. The first grating 20then comprises a first group of mutually parallel lines 21, extendingperpendicular to the first direction of measurement. The direction ofthe parallel lines 21 of the first group and the first direction ofmeasurement together define a first plane. The first reading head 8 isadapted to send a first polarized beam 15 of radiation to the firstgrating 20 in a direction perpendicular to the first plane.

For example, when displacement of the substrate table 2 relative to theprojection system 1 in x-direction is to be measured, and the firstgrating 20 is mounted onto the projection system 1 (that is: at adistance from the substrate table 2 in z-direction). In that case, thefirst direction of measurement is the x-direction and the lines of thegrating extend in y-direction. Together, the first direction ofmeasurement (x-direction) and the direction of the lines 21 of the firstgrating 20 (y-direction) define a first plane (x-y-plane). The polarizedbeam 15 of radiation the sensor head generates is directed inz-direction. This is illustrated in FIG. 6.

Instead of being mounted to the projection system 1, it is also possiblethat the first grating 20 is mounted on a frame that is substantiallystationary to the projection system 1, such that the first grating 20 isspaced from the substrate table 2 in y-direction. In that case, thelines 21 of the first grating 20 extend in z-direction.

In the lithographic apparatus according to the invention, themeasurement system may also comprises a first z-sensor 30. The z-sensor30 and the first grating 20 cooperate in determining the displacement ofthe substrate table 2 relative to the projection system 1 in a thirddirection of measurement. This third direction of measurement isperpendicular to the plane defined by the first direction of measurementand the direction of the lines of the first grating 20. So, when thefirst direction of measurement is the x-direction and the parallel lines21 of the first grating 20 extend in y-direction, the third direction isthe z-direction. So, in the lithographic apparatus according to theinvention, if a first grating 20 that is used for measuring thedisplacement of the substrate table 2 in either x-direction ory-direction, and sensor unit 9 is mounted on top of substrate table 2,adapted to send a polarized beam 15 in z-direction to first grating 20,the first grating 20 is used for measuring displacement of the substratetable 2 in the x- or y-direction as well as in the z-direction.

In a possible embodiment, the z-sensor 30 is an interferometer. In thatcase the first grating 20 has a reflective face cooperating with theinterferometer. It is envisaged that the grating comprises two, mutuallyparallel reflective faces, for example due to the nature of the grating.This can be the case if the grating comprises two groups of parallellines. In such a case, the distance between the reflective faces ispreferably n times the wavelength of the beam 15 of the interferometer,n being a positive integer (1, 2, 3, 4, . . . ).

Alternatively, the z-sensor 30 can be a capacitive sensor. Forfunctioning, a capacitive sensor needs an electrically conductivecounter plate, that is arranged at a small distance, such as a fewmillimeters, from the capacitive sensor in the direction of measurement.By making at least a part of the first grating 20 electricallyconductive, the first grating 20 serves as a counter plate for thecapacitive sensor. It is known to make gratings out of glass, with linesof chromium. By interconnecting the lines and connecting them to ground,such a grating can be used with a capacitive sensor.

The embodiment of FIG. 6 can be adapted for measuring displacement ofthe substrate table 2 relative to the projection system 1 in both thex-direction and the y-direction. Such an adapted embodiment is shown inFIG. 7. The sensor unit 9 in the embodiment of FIG. 7 comprises a secondreading head 14, which sends a second polarized beam 16 of radiation tothe first grating 20. In order to measure the displacement in theY-direction, the grating should also have a periodically alternatingpattern in the Y-direction. The first grating 20 comprises a secondgroup of mutually parallel lines 22. The parallel lines 22 of the secondgroup extend in x-direction. Thus, a two-dimensional first grating 20 iscreated. Instead of a first grating 20 of the type shown in FIG. 5, atwo-dimensional first grating with a checkerboard pattern can beapplied. The transitions between the dark and the light areas of thepattern function in the way as the mutually parallel lines 21, 22 of thefirst grating 20 according to FIG. 5. In order to provide informationregarding the position of the grating relative to the reading head, thegrating can be equipped with any alternating or varying pattern.Examples of such patterns are parallel lines, alternating patterns oflight and dark areas, areas with different colors or differentproperties with respect to reflection or absorption of the beam appliedby the sensor unit.

As an alternative for the embodiment of FIG. 7, a first and a secondsensor unit 9, 19 may be used. In that case, the first sensor unit 9comprises the first sensor head 8, and the second sensor unit 19comprises the second sensor head 14.

In an alternative embodiment, the measurement system can be such thateach sensor head 8, 14, either in the sensor unit 9 according to FIG. 7or in the alternative embodiment with a separate first and second sensorhead, cooperates with a separate, dedicated grating.

While in the embodiments of FIGS. 6 and 7, the grid grating is mountedto the projection system 1 or to a reference frame that is substantiallystationary with respect to the projection system 1, it will beappreciated that the grid grating may also be mounted to the substratetable 2. In this implementation, the sensor unit 9 including the readingheads 14 and 16 and the z-sensor 30 are mounted to the projection system1 or to the reference that is substantially stationary with respect tothe projection system 1.

In another embodiment, the measurement system is adapted to measuredisplacement of the substrate table 2 relative to the projection system1 in all three degrees of freedom in the x-y-plane (translation inx-direction, translation in y-direction and rotation about the z-axis,which rotation is referred to as the Rz-direction). To this end, themeasurement system comprises a first, a second and a third sensor head8, 14, 18. Two of the sensor heads detect displacement of the substratetable 2 relative to the projection system 1 in the same direction (forexample the x-direction), and the other sensor detects displacement ofthe substrate table 2 relative to the projection system 1 in the otherdirection (in the give example: the y-direction). The sensor heads 8,14, 18 are arranged at known relative distances, so that any rotation ofthe substrate table 2 around an axis extending in the third direction ofmeasurement can be determined based on the sensor head measurements.

It is envisaged that the first, second and third sensor heads 8, 14, 18are accommodated in a single sensor unit 9, as shown in FIG. 8 a andFIG. 8 b. In FIG. 8 a, the sensor unit 9 comprises two sensor heads 8,18 for measuring displacement of the substrate table 2 relative to theprojection system 1 in x-direction, and one sensor head 14 for measuringdisplacement of the substrate table 2 relative to the projection system1 in y-direction. In the embodiment of FIG. 8 b, the sensor unit 9comprises one sensor head 14 for measuring displacement of the substratetable 2 relative to the projection system 1 in x-direction, and twosensor heads 8, 18 for measuring displacement of the substrate table 2relative to the projection system 1 in y-direction.

Another embodiment is shown in FIG. 9. In the embodiment of FIG. 9, themeasurement system comprises a first sensor unit 9 and a second sensorunit 19. The first sensor unit 9 comprises a first sensor head 8 and asecond sensor head 14, while the second sensor unit 19 comprises a thirdsensor head 18 and a fourth sensor head 17. The sensor heads 8, 14 ofthe first sensor unit 9 cooperate with a first grating 20. The sensorheads 18, 17 of the second sensor unit 19 cooperate with a secondgrating 25. Both the first and the second grating 20, 25 are providedwith a checkerboard pattern or with a first and a second group ofmutually parallel lines, so that both sensor units 10, 19 are adapted tomeasure displacement of the substrate table 2 relative to the projectionsystem 1 in x-direction and in y-direction.

This embodiment may be particularly useful in view of potentialimprovements in the resolution of the measurement of the rotationaldisplacement around the z-axis of the substrate table 2 relative to theprojection system 1.

FIG. 10 shows an embodiment in which a first sensor unit 9, comprising afirst sensor head 8 for measuring displacement in x-direction, and asecond sensor head 14 for measuring displacement in y-direction, iscombined with a third sensor head 18 for measuring displacement iny-direction and a fourth sensor head 17 for measuring displacement iny-direction. All sensor heads 8, 14, 18, 17 cooperate with atwo-dimensional first grating 20. In the arrangement of FIG. 10,displacements of the substrate table 2 in x-, y- and Rz-direction aremeasured using the first sensor unit 9 and either the third of thefourth sensor head 17, 18, depending on which one has a beam ofradiation in contact with the first grating 20.

FIG. 11 shows an embodiment in which the first grating 20 is attached toa frame 5 instead of directly to the projection system 1. Preferably,this frame 5 is at least substantially stationary relative to theprojection system 1, that is: the relative displacement of the part ofthe frame 5 the grating is attached to and the projection system 1 isnegligible. If the relative displacement of the part of the frame 5 thegrating is attached to and the projection system 1 is not negligible,the lithographic apparatus is preferably provided with an additionalsensor 35. This additional sensor 35 determines the relativedisplacement of the part of the frame 5 the grating is attached to andthe projection system 1. This relative displacement is then taken intoaccount when the displacement of the substrate table 2 relative to theprojection system 1 is determined.

In FIG. 11, the additional sensor 35 is mounted to the frame of theprojection system 1, which may also be referred to as the “lens frame”since it includes a lens of the projection system. The frame 5 may alsobe termed a “measurement frame”.

It will be appreciated that the scope of the invention is not limited bythe above described embodiments. For example, various combinations ofthe above described embodiments could be used to measure (a)displacements of the lens frame or projection system with respect to theframe 5 and (b) displacements of the frame 5 with respect to thesubstrate table, and vice and versa. Specifically, it will beappreciated that the sensor 35 could be arranged either on theprojection system or on the frame 5. In such a case, the sensor 35 willcooperate with the grating mounted to the frame 5 or to the projectionsystem. In addition, the sensor mounted to the substrate table could bearranged either on the substrate table or on the frame 5. In such acase, the sensor will cooperate with a grid grating mounted to the frame5 or to the substrate table. The grid grating that cooperates with thesensor mounted to the substrate table could be the same as the gratingthat cooperates with the sensor 35.

Specifically, as shown in FIG. 11, the relative displacement between thesubstrate table 2 and the projection system 1 is carried out with thegrating attached to the frame 5 and the sensor head mounted to thesubstrate. In this implementation, the same grid grating cooperates with(a) the additional sensor 35 to measure displacements between theprojection system 1 and the frame 5 and with (b) the sensor mounted tothe substrate table to measure displacements between the substrate tableand the frame 5.

Alternatively, it will be appreciated that a different grating could beused to measure displacements between the substrate table and the frame5. In this implementation, a second grid grating may be mounted to theframe 5 and is configured to cooperate with the sensor mounted to thesubstrate table.

Furthermore, it will also be appreciated that displacements between theprojection system 1 and the frame 5 may be measured with the gridgrating mounted to the projection system 1 and the additional sensor 35mounted to the frame 5.

In another embodiment, the second grid grating may be mounted to thesubstrate table. In such a case, the sensor that cooperates with thesecond grid grating is mounted to the frame 5 instead of the substratetable. Displacements between the lens frame and the frame 5 ormeasurement frame may be measured, for example, with the sensor 35 andthe grid grating arranged on the frame 5.

In an implementation, displacements between the projection system andthe frame 5 may be measured with a first grating arranged on theprojection system and the sensor 35 mounted to the frame 5.Displacements between the lens frame 5 and the substrate table may bemeasured with a second grating mounted on the frame 5 and a secondsensor mounted on the substrate table. The second sensor and the secondgrating cooperate with each other to provide the measurements.

While in the above described embodiments of FIG. 11, the apparatus isconfigured to measure displacements of the substrate table, similarmeasurements could be carried out for the mask table. Specifically,displacements between the projection system 1 and a measurement frame 5could be measured in a same way as in FIG. 11. Displacements between themeasurement frame 5 and the mask table could also be carried out in thesame was as in the above described embodiments.

In that respect, it will be appreciated that various combinations of theabove described embodiments could be used to measure (a) displacementsof the lens frame or projection system with respect to the frame 5 and(b) displacements of the frame 5 with respect to the mask table, andvice and versa. Specifically, it will be appreciated that the sensor 35could be arranged either on the projection system or lens frame or onthe frame 5. In such a case, the sensor 35 will cooperate with thegrating mounted to the frame 5 or to the projection system or lensframe. In addition, a sensor could be arranged either on the substratetable or on the frame 5. In such a case, the sensor will cooperate witha grid grating mounted to the frame 5 or to the mask table. The gridgrating that cooperates with the sensor mounted to the mask table couldbe the same as the grating that cooperates with the sensor 35.

It is envisaged that the embodiments shown in the FIGS. 2 to 11 can beused without the presence of a z-sensor that cooperates with a grating.

In another embodiment (shown in FIG. 12), the measurement systemcomprises a first, a second and a third z-sensor 31, 32, 33. Thesez-sensors 31, 32, 33 are arranged at known relative positions, so thatany rotation of the substrate table 2 around the x-axis (Rx) and aroundthe y-axis (Ry) can be determined by their measurements. Preferably allz-sensors cooperate with the first grating 20 in measuring thedisplacements of the substrate table 2 relative to the projection system1. It is however envisaged that is it possible that different z-sensors31, 32, 33 cooperate with different gratings. The z-sensors 31, 32, 33of the embodiment of FIG. 12 can be combined with any of the embodimentsshown in FIGS. 2-11.

While we have described above specific embodiments of the invention itwill be appreciated that the invention may be practiced otherwise thandescribed. The description is not intended to limit the invention. Inparticular, while the described embodiment is a system for measuring theposition of a mask table in a lithographic apparatus, it will beappreciated that the invention is equally applicable to substrate(wafer) tables and to multiple stage; devices. Also, the grid gratingcan be mounted on a fixed part of the apparatus, such as a metrology orreference frame, and the sensor head can be mounted on the moveableobject.

1. A lithographic apparatus comprising: a support structure configuredto support a patterning device, the patterning device configured topattern a beam of radiation; a substrate table configured to hold asubstrate; a projection system configured to project the patterned beamonto a target portion of the substrate; and a displacement measurementsystem having a first sensor unit and a second sensor unit andconfigured to measure a displacement of a moveable object in two degreesof freedom relative to a reference frame, the movable object comprisingone of the support structure and the substrate table, wherein thedisplacement measurement system further comprises two grid gratingsmounted on the reference frame, each grid grating corresponding to oneof the first and second sensor units; and the first and second sensorunits are mounted on the moveable object and configured to measuredisplacements of a respective one of the grid gratings.
 2. Thelithographic apparatus of claim 1, wherein the reference frame comprisesthe projection system.
 3. The lithographic apparatus of claim 1, whereineach of the first and second sensor units comprises a pair of sensorheads.
 4. The lithographic apparatus of claim 1, wherein the measurementsystem further comprises a Z-sensor configured to measure at leastanother degree of freedom of the moveable object relative to thereference frame.
 5. The lithographic apparatus of claim 3, wherein eachof the sensor heads includes an encoder.
 6. The lithographic apparatusof claim 4, wherein the Z-sensor comprises: an interferometer systemconfigured to co-operate with a reflective surface of a grid grating, ora capacitive sensor system configured to co-operate with a conductivepart of a grid grating.
 7. A lithographic apparatus comprising: asupport structure configured to support a patterning device, thepatterning device configured to pattern a beam of radiation; a substratetable configured to hold a substrate; a projection system configured toproject the patterned beam onto a target portion of the substrate; and adisplacement measurement system configured to measure a displacement ofa moveable object in at least two degrees of freedom relative to areference frame, the movable object comprising one of the supportstructure and the substrate table, and the displacement measurementsystem having a first sensor head and a first sensor unit comprising apair of sensor heads, wherein the displacement measurement systemfurther comprises a grid grating mounted on the moveable object; and thefirst sensor head and the first sensor unit are mounted on the referenceframe and configured to measure a displacement of the grid grating. 8.The lithographic apparatus of claim 7, wherein each of the sensor headsincludes an encoder.
 9. The lithographic apparatus of claim 7, whereinthe measurement system further comprises a fourth sensor head.
 10. Alithographic apparatus comprising: a support structure configured tosupport a patterning device, the patterning device configured to patterna beam of radiation; a substrate table configured to hold a substrate; aprojection system configured to project the patterned beam onto a targetportion of the substrate; and a displacement measurement system having afirst sensor unit and a second sensor unit and configured to measure adisplacement of a moveable object in two degrees of freedom relative toa reference frame, the movable object comprising one of the supportstructure and the substrate table, wherein the displacement measurementsystem further comprises two grid gratings mounted on the moveableobject, each grid grating corresponding to one of the first and secondsensor units; and the first and second sensor units are mounted on thereference frame and configured to measure displacements of a respectiveone of the grid gratings, wherein at least one of the two grid gratingsinclude a reference mark detectable by the respective sensor unit todefine a reference position of the movable object.
 11. A lithographicapparatus comprising: a support structure configured to support apatterning device, the patterning device configured to pattern a beam ofradiation; a substrate table configured to hold a substrate; aprojection system configured to project the patterned beam onto a targetportion of the substrate; and a displacement measurement systemconfigured to measure a displacement of a moveable object in at leasttwo degrees of freedom relative to a reference frame, the movable objectcomprising one of the support structure and the substrate table, and thedisplacement measurement system having a first sensor head and a firstsensor unit comprising a pair of sensor heads, wherein the displacementmeasurement system further comprises a grid grating mounted on thereference frame; and the first sensor head and the first sensor unit aremounted on the moveable object and configured to measure a displacementof the grid grating.